feat: implement spire descent system with room-aware navigation

Implements the spec-driven spire multi-room climbing and descent system:
- Room navigation: currentRoomIndex, roomsPerFloor, startFloor, exitFloor
- Descent tracking: descentPeak, roomResetState, clearedRooms, isDescentComplete
- enterDescentMode: snapshots peak, sets climbDirection='down'
- advanceRoomOrFloor: room-by-room ascending/descending with floor transitions
- onEnterRoomDescend: per-room 50% reset check with auto-skip
- onEnterLibraryRoom: discipline XP scaled by floor
- Seeded PRNG for deterministic room counts and types
- UI: Descend button during ascent, Exit Spire only when isDescentComplete
- UI: Room X/Y display, room type badge, in-game time in RoomDisplay
- Extracted descent actions to combat-descent-actions.ts (file size limit)
- Updated tests for room-aware combat behavior

Spec: docs/specs/spire-climbing-spec.md §4.1-§4.9, §6

src/lib/game/__tests__/cross-module-helpers.ts

@@ -44,6 +44,14 @@ export function resetAllStores() {
     clearedFloors: {},
     climbDirection: null,
     isDescending: false,
+    startFloor: 1,
+    exitFloor: 1,
+    currentRoomIndex: 0,
+    roomsPerFloor: 5,
+    descentPeak: null,
+    roomResetState: {},
+    clearedRooms: {},
+    isDescentComplete: false,
     golemancy: { enabledGolems: [], summonedGolems: [], lastSummonFloor: 0 },
     equipmentSpellStates: [],
     comboHitCount: 0,